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WIREs Nanomed Nanobiotechnol
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Smart dual‐mode fluorescent gold nanoparticle agents

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Fluorophore‐mediated, molecular sensing is one of the most popular and important technique in biomedical studies. As in any sensing technique, the two most important factors in this sensing are the sensitivity and specificity. Since the fluorescence of a fluorophore is emitted in the process of fluorophore electrons returning from their excited to ground state, a tool that can locally manipulate the electron state can be useful to maximize the sensitivity and specificity. A good tool candidate for this purpose is nanosized metal particles that can form an electromagnetic (EM) field at a sufficiently strong level, upon receiving a particular wavelength that fits the excitation wavelength of the fluorophore to be used. There are several metal nanoparticle types that can generate a sufficiently strong EM field for this purpose. Nevertheless, for the biomedical studies, which require minimal toxicity, gold nanoparticles (GNPs) are known to be the most suitable. In this article, various methods for fluorescence alteration using GNPs, which can be beneficially utilized for biomarker‐specific, highly sensitive molecular sensing and imaging, are discussed. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > In Vitro Nanoparticle-Based Sensing
Theoretical estimation of fluorescence output (Φ) of various fluorophores, affected by a 10, 30, or 50 nm GNP, with respect to the distance from the GNP surface. The black, horizontal dashed line at Φ = 1 shows the fluorescence level without GNP. The numbers in the parentheses next to the fluorophore names are the wavelengths of the excitation and emission peaks for the respective fluorophores.
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Levels of Cypate fluorescence affected by (a) the concentration of Cypate itself and (b) the number of free GNPs in the solution.
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Fluorescence levels of Cypate with respect to the ionic strength of the solution.
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Schematic diagrams of keeping a constant distance of the fluorophore from the surface of GNP. (a) A polymer layer at a desired thickness for fluorescence enhancement is placed on the GNP surface and the fluorophore is placed on the surface of the polymer layer. (b) Fluorophores are placed at one end of a linear spacer with the length providing fluorescence enhancement and the other end of the spacer is immobilized on the GNP surface.
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Relative fluorescence of the GNP–Cypate complex linked via a short spacer containing uPA substrate, for the core GNP size of 3.7, 8.0, and 16.4 nm. For the GNP sizes of 3.7 and 8.0, fluorescence is quenched extensively but for 16.4 nm GNP, fluorescence is enhanced.
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A schematic diagram of examples to use the GNP fluorescence quenching ability to enhance specificity of sensing: (a) When a fluorescing biomarker meets a GNP‐linked anti‐biomarker fluorescence quenches and it can be used for negative signal sensing or sensing by competitive reaction; (b) a hairpin‐shaped nucleotide sequence containing a nucleotide section complementary to a biomarker nucleotide, as in molecular beacon but the quencher is a GNP. When it meets the biomarker nucleotide the hairpin stretches out and the fluorescence gets restored; and (c) a short spacer containing an enzyme substrate motif is linked to a GNP on one end and a fluorophore, on the other end. Normally, little fluorescence is emitted. When the complex is placed in a sample containing the enzyme biomarker the spacer is cleaved, the fluorophore is released, and the fluorescence is restored.
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A qualitative description of the fluorescence level change of a fluorophore influenced by a GNP at various sizes, with respect to the distance from the surface. For all GNP sizes, there are ranges that quench and enhance fluorescence. In general the maximum enhancement level is greater for the larger particle size.
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